feat(translations): add all posts, all categories, all tags

[asierrayk]

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• My Pull Request is filled against the next branch
• All commits in my Pull Request conform to Elegant Git commit Guidelines

Recommended Steps

• My patch adds a new feature, therefore I have also added a help article about it
• My patch changes Elegant behavior, therefore I have updated the help article to reflect this change
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Description

Add translations for All Posts, All Categories and All Tags

Putting the follwing code in the pelicanconf.py:

ALL_POSTS = 'Publicaciones'
ALL_CATEGORIES = 'Categorías'
ALL_TAGS = 'Etiquetas'